1. Field of the Invention
The present invention relates to a catalytic system and the relative process for the oxidative dehydrogenation of alkylaromatics, in particular ethylbenzene, or of paraffins to the corresponding alkenylaromatics, in particular styrene, or to the corresponding olefins.
2. Description of the Background
Styrene, which is an important intermediate for the production of plastic materials, is mainly used in the production of polystyrenes (GPPS crystals, shock-resistant HIPS and expandable EPS), acrylonitrile-styrene-butadiene (ABS) copolymers and styrene-acrylonitrile (SAN) copolymers of styrene-butadiene rubbers (SBR).
At present, styrene is mainly produced by means of two processes: by the dehydrogenation of ethylbenzene (EB) and, as coproduct, in the epoxidation of propylene with ethylbenzene hydroperoxide with catalysts based on molybdenum complexes.
An alternative method for the production of the monomer is the dehydrogenation of ethylbenzene with the contemporaneous oxidation of hydrogen which can be carried out in the presence of or without oxygen.
Oxidative dehydrogenation without oxygen consists in the use of one or more metal oxides which, apart from catalyzing the dehydrogenation reaction of ethylbenzene, are capable of oxidizing, by means of the oxygen available on the oxide itself, the hydrogen produced, in order to favor the equilibrium shift towards the formation of styrene (STY) by means of the reaction 
It is evident from the reaction (1) that the catalyst also participates in the stoichiometry of the reaction, by acting as reagent: at the beginning of the reaction it is in an oxidative state (catox) capable of giving part of its oxygen and becoming a reduced species (carred). In order to make the reaction catalytic it is necessary for the reduced catalyst to be able to easily recuperate the oxygen to be transformed into the starting oxidized species, useful for a new oxidative dehydrogenation cycle, by means of the reaction 
This particular way of conducting the dehydrogenation has the same advantages as the traditional oxidative dehydrogenation, i.e. In the presence of oxygen, allowing the heat necessary for the dehydrogenation to be produced and the equilibrium of the dehydrogenation reaction to be shifted towards the products, without disadvantages such as the by-production of oxygenated compounds deriving from the use of an oxidation gas.
The idea of carrying out the oxidative dehydrogenation of hydrocarbons without an oxidating gas was already made known in the first half of the 60s"" by U.S. Pat. No. 3,118,007 of Bayer. This patent claims a process for the dehydrogenation of hydrocarbons without oxidating gases and with catalysts based on iron oxides which also act as oxygen transporters. It also describes the possibility of operating under fluid bed conditions to allow the continuous removal of the catalyst to be subjected to a reoxidation phase and subsequent recycling to the reaction phase.
Various patents on oxidative dehydrogenation without oxidating gases have been filed in the last few years, of which the most pertinent are the following.
EP-482276 of FINA describes a process by which a total conversion of ethylbenzene is already obtained at 505xc2x0 C. with a catalyst which acts as oxygen transporter and which once exhausted, can be regenerated in a second reactor by treatment with air. The catalyst, containing oxides of transition metals, preferably based on vanadium supported on magnesium, has a high dehydrogenating activity as well as a strong tendency to provide structural oxygen for the combustion of the hydrogen. The results specified in this patent demonstrate that combustion is the most critical phase of the reaction: at the beginning of the catalytic activity, in fact, the styrene is produced with a low selectivity together with a high quantity or carbon oxides deriving from the combustion of ethylbenzene and/or styrene. The same patent shows that a partial pre-reduction of the catalyst, by treatment with carbon monoxide, enables its strong oxidating capacity to be moderated and high selectivities to be obtained already in the first phases of activity. In this case however, the conversion rapidly drops and soon becomes stabilized at values of approximately 50%.
GB-2297043 of BASF claims the use of a catalyst consisting of a mixed oxide based on bismuth, titanium, lanthanum, potassium and treated with a noble metal, for the oxidative dehydrogenation of ethylbenzene without oxygen. The results indicated do not allow the catalytic performances to be accurately evaluated over a period of time. In the text of the patent it is stated that the catalyst is initially very active but not very selective with the formation of compounds deriving from the combustion of hydrocarbons. As already observed in the case of the FINA patent, as the process proceeds the catalyst becomes less active and increasingly more selective until it reaches the maximum value.
We have surprisingly found that with the use of catalysts based on vanadium and bismuth suitably supported, with respect to the know catalysts described above, in addition to there being better selectivity characteristics, mainly at the beginning of the reaction, and a higher total productivity, the duration of the life of the catalyst itself is also greater.
The catalytic system of the present invention for the Om oxidative dehydrogenation of alkylaromatics (in particular ethylbenzene) or paraffins to the corresponding alkenylaromatics (in particular styrene) or to the corresponding olefins, consists of:
a vanadium oxide;
a bismuth oxide;
and a carrier based on magnesium,
wherein
the vanadium, expressed as V2O5, is in a quantity ranging from 1 to 15% by weight, preferably from 2 to 10%,
the bismuth, expressed as Bi2O3, ranges from 2 to 30% by weight, preferably from 5 to 25% by weight, the complement to 100 being the carrier.